Sunday, December 8, 2013

Wild Snow on the Tug Hill Plateau

On average, the lake-effect snow belts of the eastern United States receive some of the driest snow in the United States, comparable to that found in the interior west, including Utah and Colorado.

We got the opportunity to sample what meteorologists call wild snow on the Tug Hill Plateau yesterday and this morning as part of our efforts for the National Science Foundation sponsored Ontario Winter Lake-effect Systems (OWLeS) field program. Wild snow is extremely dry snow with a water content of 5% or less. For comparison, the average water content of snow that falls in Utah has a water content of 8.5%. Heavy snow that is difficult to shovel typically has a water content of 12% and sometimes exceeds 20%.

Although we have a bunch of fancy automated instruments to measure snow, I'm partial to making snow depth, snow-water equivalent (SWE - the amount of water in the snow), and water content measurements by hand. The technique is old school, but more reliable than automated measurements. It involves placing out a white snowboard, letting the snow accumulate, and then making measurements with a ruler.

That gives us the snow depth for whatever period during which the snow accumulates on the board. We then take a core of that snow and weigh it to determine the SWE. Dividing the SWE by the snow depth gives you the water content.

Using this approach we typically measure the snow depth, SWE, and water content every 6 hours, although we sometimes go 12 hours overnight when the snowfall isn't very large (as was the case last night). We also do a 24-hour measurement on a separate board each morning.

Over the past 24 hours, we've received 13 cm (about 5 inches) of snow with a water content of only 4%. That's really dry snow. There are two reasons why it is so dry. The first is the type of snowflakes that are falling. As shown in the photo below, we've observed a mixture of crystal types. The branchy tree-like flakes are dendrites. When these crystals entwine to make bigger flakes, which you can see a few of in the photo below, we call them dendritic aggregates. There are also some smaller more rounded crystals.

Dendrites and dendritic aggregates are a key ingredient in wild snow because all those tree-like arms create lots of air pockets as the flakes stake up on top of each other.

The second reason is a lack of wind. Wind tends to bash and break up dendrites, destroying all those air pockets. So, if you want wild snow, you need calm or near calm conditions so the dendrites can fall unmolested and stack up like a house of cards.

Five inches isn't a lot of snow, but it's added just enough to the base that we will be skiing later today. Its also served as an appetizer for the main course that will be coming this week when it appears we will need to get out our meter stick for measurements.

3 comments:

Just tried to get a melted core value for 8" of new snow here in the SLC area (Taylorsville) and came up with about 0.48" (compared to 0.35" gage catch). I think the 0.48" is a more accurate number so maybe around 6% density. In any case it is very dry.

Ah, thanks for the explanation on the fluffy snow out east. I fondly remember the light, fluffy, dendritic flakes puffing up and falling gently around my cross country skis on our first ski out in the 'Dacks. Beautiful snow for a west coast transplant, but we're really glad to be playing in Wasatch snow now!

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The Wasatch Weather Weenies discuss the weather and climate of the Wasatch Front and Mountains, western United States, and beyond.

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